This invention relates to the preparation of stable, homogeneous melt solutions useful for the preparation of homopolycarbonates and copolycarbonates.
Polycarbonates are widely used, relatively low cost thermoplastics esteemed for their combination of clarity, toughness, heat resistance and moldability. The most widely used polycarbonate is bisphenol A polycarbonate, hundreds of millions of pounds of which are manufactured and sold annually on a world wide basis.
Polycarbonates, both homopolycarbonates and copolycarbonates, are advantageously prepared in the absence of a solvent in what is sometimes referred to as the “melt process” in which a diaryl carbonate and one or more bisphenols are heated in a molten state in the presence of a transesterification catalyst. This melt method offers certain advantages over the widely used interfacial method of polycarbonate preparation which employs a solvent, typically methylene chloride. Methylene chloride is a low boiling chlorinated hydrocarbon and its use necessitates costly engineering measures to prevent its adventitious introduction into the environment. The melt process as a general proposition is less demanding with respect to controlling emissions since no solvent is involved and the only major volatile by-product produced, phenol, is quite high boiling (bp 182° C.) and is thus less difficult to contain using ordinary engineering means, such as water cooled condensers. The melt process suffers several disadvantages however, chief among them is the need to heat the reactants to relatively high temperatures 250-320° C. under low pressure, about 180 to about 0.1 mbar. The use of high temperatures is nearly always accompanied by unwanted side reactions which take place during the polymerization reaction. Most notable among unwanted side reactions taking place in the melt process is the Fries reaction. The presence of Fries reaction products in the product polycarbonate may negatively impact the color, stability and flow properties of the product polycarbonate.
In certain instances when preparing polycarbonates by the melt method it is found convenient to prepare a molten mixture of the reactants used in the melt process, a diaryl carbonate and a dihydroxy aromatic compound, which is then introduced by gravity flow or through a pump capable of pumping fluids into a melt reactor or series of melt reactors. This presents few difficulties when the diaryl carbonate and dihydroxy aromatic compound have low melting points, but may present serious engineering challenges where the melting point of the either the diaryl carbonate or the dihydroxy aromatic compound is in excess of 200° C. Where a mixture of diaryl carbonate and dihydroxy aromatic compound comprises a high melting bisphenol, for example 4,4′-biphenol, it is frequently difficult to achieve a homogeneous melt for use as a feedstock in the melt polymerization reaction. It would be highly desirable to discover a means of producing stable, homogeneous melt solutions from mixtures of diaryl carbonates and high melting dihydroxy aromatic compounds which are stable with respect to resolidification of the melt solution and with respect to precipitation of individual components comprising the melt solution. Moreover it would be highly desirable to discover a means of producing stable, homogeneous melt solutions from mixtures of diaryl carbonates and dihydroxy aromatic compounds having melting points in excess of 200° C., at temperatures substantially below the melting points of the highest melting constituent dihydroxy aromatic compound.